An optical material has been known, which control the amount of light transmission and a light scattering property by using a polymer gel, whose volume is reversibly changed (swelling and contraction) in response to the change in pH, the change in ion concentration, the adsorption, desorption of a chemical substance, heat, light, electric stimulus, or the like.
For example, JP-A-61-151621, JP-A-62-925 and JP-A-4-134325 disclose a device utilizing a polymer gel whose volume is changed by absorbing or releasing a liquid caused by a temperature change or an electric stimulus, the device displaying or erasing an image by changing the refractive index of the polymer gel by changing its density. JP-A-5-188354 discloses an optical device utilizing polymer gel particles whose volume is changed by absorbing or releasing a liquid caused by a stimulus of an electric field, the optical device becoming white turbid or transparent by making the diameter of the polymer gel particles larger or smaller than the wavelength of light to change the refractive index.
However, in these optical devices disclosed in these publications, because the usable materials of the polymer gels are restricted, the difference in refractive index between the polymer gel and the liquid cannot be made large, and, as a result, a contrast between the background (white turbid part) and the image (transparent part), i.e., the display contrast, is low.
JP-A-61-149926 and JP-A-7-95172 disclose an optical device constituted by a composition comprising a composition containing a polymer gel whose volume is changed by absorbing or releasing a liquid caused by a stimulus of an electric field, and a liquid colored with a coloring matter. In this optical device, the colored liquid moves by the change in shape of the polymer gel, to display or erase an image by the change in light absorption.
However, in these optical devices disclosed in these publications, the colored liquid is mixed in the polymer gel by the repetition of swelling and contraction of the polymer gel, to thereby contaminate the polymer gel, and, as a result, there is a possibility that the display contrast is lowered.
JP-A-61-151625 and JP-A-62-927 disclose a device using a colored polymer gel which swells or contracts by heat. This device utilizes such a mechanism that when the polymer gel swells, i.e., the volume of the polymer gel is increased, the concentration of the coloring matter is decreased to lower the optical density, and when the polymer gel contracts, the concentration of the coloring matter is increased to increase the optical density.
However, in the optical device disclosed in the above publication, because the polymer gel is colored when the volume thereof is small, a sufficient display contrast cannot be obtained.
JP-A-4-274480 discloses an optical device utilizing a polymer gel, to which a dye is connected by a covalent bond, swelling or contracting by an electric field. The optical device utilizes such a mechanism that when the polymer gel swells, i.e., the volume of the polymer gel is increased, the optical density is increased by increasing the area of the polymer gel capable of absorbing light, and when the polymer gel contracts, i.e., the volume of the polymer gel is decreased, the optical density is lowered by decreasing that area.
However, in the optical device proposed in the above publication, a sufficient display contrast cannot be obtained because the concentration of the dye contained in the polymer gel is low.
JP-A-9-160081 discloses an optical device utilizing colored fine particles and a polymer gel adsorbed on the colored fine particles. In this optical device, the polymer gel swells or contracts by the action of an electric field, and the optical density is changed by the change in the ratio of that surface of the colored fine particles which is covered with the polymer gel.
However, in the optical device disclosed in the above publication, when the used amount of the polymer gel is increased with respect to the amount of the colored fine particles, the optical density is not sufficiently increased when the polymer gel contracts, and the display contrast is lowered. When the used amount of the polymer gel is decreased, on the other hand, the optical density is not sufficiently increased when the polymer gel swells, and the display contrast is lowered.
The inventors have proposed a novel optical material in Japanese Patent Application No. 9-345541. The optical material disclosed in the above application comprises a polymer gel which swells or contracts by absorbing or releasing a liquid by providing a stimulus and contains a pigment in a concentration larger than the saturated absorbing concentration. In the optical material, when the polymer gel contracts, the pigment locally agglomerates to lower the light absorption efficiency and the optical density, so as to cause discoloration. When the polymer gel swells, on the other hand, the light absorption efficiency is increased by dispersing the pigment throughout the optical material, and the optical density is increased to cause color generation.
However, in the optical material disclosed in the above application, because the agglomerated pigment contained in the polymer gel in a high concentration absorbs light even when the polymer gel contracts, the optical material cannot be completely discolored.